The noxious invasive weed whitetop, which has colonized vast tracts of the West with its pale flower clusters, has long confounded landowners and resource managers. Also known as hoary cress, the plant proliferates quickly, has no natural North American predators and is reportedly toxic to cattle in large quantities.
An unlikely warrior has emerged in the whitetop battle: a microscopic mite from Europe researchers believe can stunt the weed’s growth. This summer, land managers released the mite in Wyoming for the first time.
“We’re really excited about having the mites,” said Dr. Tim Collier, a University of Wyoming associate professor who specializes in biological control of rangeland weeds. “Whitetop is a huge problem in Wyoming. And so we’ve been really eager to get them.”
Technicians spread mite-infested “galls” — plant deformities caused by feeding mites — on a 3.5-acre piece of state-owned property in Fremont County in May. The hope is the mites will spread to live plants and create new galls, which impede growth.
A journey around the world
When Dr. Jeffrey Littlefield joined a Zoom call from his facility at Montana State University in Bozeman, he was clad in scrubs, his hair under a protective cap and his feet in booties. This is typical; working with invasive species requires rigorous containment measures.
Littlefield, a MSU research scientist, is a major player in the whitetop mite effort, a collaboration many years in the making that spans the globe.
The plant-feeding gall mite in question, which is nearly too small for the naked eye to see, hails from northern Greece. Aceria drabae is distinguished by its reduced number of legs, which are located near its head, and ornately feathered claws.
“It’s been known for a number of years and was thought to be a potential bio control agent for whitetop,” Littlefield said.
A biological-control laboratory began transporting the tiny creature to the containment facility at MSU in the mid-’90s, Littlefield said. That effort entailed field scientists in Greece collecting plant bouquets infected with galls, then shipping the material halfway across the world, hoping they arrived with enough live mites to work with.
In order to transport the diminutive creatures individually (a measure to eliminate contamination of unwanted organisms), Littlefield and his researchers used brushes made from a single eyelash glued to a meat skewer. “They are pretty tiny, so a lot of our work is done under a scope,” he said.
Littlefield estimated his team transferred thousands of mites to establish lab colonies.
Over the years of transfer, testing and monitoring, researchers found positive results. The mites produce different galls — from forms that resemble little heads of broccoli to larger deformities — that prevent the plants from going to seed, Littlefield said. Galls also spread to secondary stems and forced their host plants to divert energy that would otherwise be used to proliferate. “They can really stunt the plant,” he said.
Littlefield’s team also tested different methods of mite transfer to determine what’s most effective.
“The critical part is trying to find plants in the right stage of development,” he said. “It took a couple years to kind of figure out exactly what stage to inoculate.” [Answer: the vegetative stage.]
Littlefield and his team also had to secure USDA regulatory approval. The process for approving biological control agents — natural enemies such as parasites, predators or pathogens — is painstaking and comprehensive.
“And that took probably another good six years or so,” Littlefield said.
But that approval gave Littlefield and his collaborators the green light to release the mite on wild whitetop, which they first did in Montana in 2019.
So far, so good. The first year they put mites in one Montana site, Littlefield said, they counted 10 gall-infected stems. “This past year we’ve had well over 6,000 stems,” he said. “We’re finding not only the number of infested stems has increased, but the gall intensity has increased.”
The hope is to slowly grow the program in order to facilitate releases in more of the West and see a slowing or reversal in the weed’s colonizing patterns — “to reduce seed production, reduce the spread of the plant and hopefully impact some of the plant density,” Littlefield said.
Which is how, more than 20 years after the project began, the mite came to be released in Wyoming.
Wyoming’s whitetop problem
The Wyoming Weed and Pest Control Council has been contributing to biological control research about hoary cress since 2001, according to the agency. When the mites became available for release, the council wanted in on it.
Whitetop has, after all, affected every county in the state, Larry Smith, President of Wyoming Weed and Pest Council, said in a release. “We implement cultural practices, herbicides and grazing every year to slow the spread, but it still heavily impacts Wyoming. That’s why we need biological control to help over time.”
There are plenty of introduced plants that don’t cause ecological disorder, said Fremont County Weed and Pest District Supervisor Aaron Foster, who chairs the Wyoming Biological Control Steering Committee. “But some have the advantage of being competitive and can cause havoc. And whitetop is one of those.”
Because it’s rhizomatous and spreads by seed, mowing it down doesn’t generally work, he said.
Whitetop “has the ability to use that root system to crowd out the surrounding vegetation,” Foster said. “And it grows and it forms these dense monoculture patches. And they just get bigger and bigger and kind of expand, and start pushing out the desirable grasses, the other forbs that are in there that we like, and pretty quick, the dominant plant is whitetop.”
That’s not good for biodiversity, wildlife or agriculture, he said.
The site near Dubois was selected for a few reasons, Foster said. The county has been a longtime and significant contributor to bio control, research and development in Wyoming, he said, and county weed and pest officials already had sites identified and prepared there.
Now that the mites have been released, the next step is to wait, monitor and evaluate whether to distribute them more widely, Foster said.
This project will take years, and isn’t expected to eradicate whitetop in Wyoming. But the hope is that it can slow the spread and save money by preventing other more expensive measures.
“You’re taking that one step forward … [causing] a little bit of impact that’ll slow them down,” Foster said. That can help the larger effort “of bringing it back into a balanced system.”